Since mankind has been recording such things, 3,880 comets have been discovered and written up. And a staggering 147 million observations have been made of these comets[1]. These observations permit, amongst other things, the orbits of the comets to be determined.

As a comet moves around the Sun, little pieces may break off, while other bits of comet stuff – both gas and dust – are ejected as the comet outgasses. This “comet sweat” ends up as a trail of dusty debris – a meteoroid stream – littering the cometary orbit. The smallest bits of debris are quickly swept away by the Sun’s light and the solar wind; the larger, heavier pieces tend to remain in the comet’s trajectory for longer. If a comet’s orbit (past or current) crosses the orbit of the Earth, there is a chance that a meteor shower will be seen when these comet fragments collide with our planet, vaporizing in the upper atmosphere.

Some 20 comets are known to produce meteor showers. Of these, Halley’s Comet is the most famous, and gives us two regular meteor showers: the eta Aquariids (early in May) and the Orionids (in late-October). The only other comet that gives two showers is Comet 96P/Machholz (the Arietids and the Southern Delta Aquariids).

One researcher interested in the link between comets and meteor showers is Quan-Zhi Ye, a Ph.D. candidate at the Dept. of Physics and Astronomy, University of Western Ontario (Canada)[2]. Quan-Zhi studies demising comets and their meteoroid streams, primarily by performing numerical simulations. These models can “turn back the clock” for a comet’s path for only a relatively short interval (about 1,000 years) before the chaotic nature of the orbit makes the results unreliable. Models can also be built that use many hundreds of numerical instances (“clones”) of a given comet, and statistically analyzing the aggregated results of the clone’s behaviour.

By running the simulator’s clock forwards or backwards, predictions can be made for encounters between the Earth and a comet’s meteoroid stream. These simulations produce a set of solutions rather than a single answer, and usually selecting the best solution needs observations. For example, the model may predict past meteor activity, in which case historical records may confirm the prediction. Future-state models may predict upcoming activity which require observations to confirm or reject.

Quan-Zhi, and colleages Peter G. Brown and Paul A. Wiegert, have recently built such models for two comets, P/2016 BA14 and 252P/LINEAR. Both comets will be near the Earth later this month, and their meteoroid trails may produce some interesting results – even if these are null results.

Activity from P/2016 BA14

Comet P/2016 BA14 will pass within about 0.024 AU of the Earth on March 23[3]. Quan-Zhi ran his model for this comet from 1750 AD to the present, and found no trail encounter with the Earth around this time. However, older meteoroid trails could still produce meteor activity.

The point, of course, is that nobody knows. Only by actually looking (sorry, Aristotle) will we learn. And, of course, a good observation always yields results, and something always happens. What happens may not be what we would like to happen: it may be that there are no meteors visible, but that is a result, too. As Quan-Zhi puts it: “you don’t see things you are not looking for,” adding that “even negative observations are useful to verify the past activity of the comet.”[4]

Quan-Zhi warns that “significant activity is quite unlikely ... because (1) there is no direct encounter to ‘visible’ meteoroids (visible all the way to low-light cameras and typical radars, brighter than +8-ish mag); and (2) the Earth will meet some debris but it is on the smaller end (fainter than +10 mag if they exist at all).”

Significantly, though, he adds: “If the comet had a catastrophic fragmentation long ago, my model wouldn’t have captured that; only meteor observations would tell us.”

“The best time [to observe] would be in the late UT hours of Mar 20, probably around 20-24h UT,” Quan-Zhi writes, adding that “this is only a rough idea of when the activity might show up. ... The radiant is near RA = 82 deg, Dec = –39 deg (J2000 epoch) [in Columba]. The meteors (if any) will be particularly slow, [geocentric velocity] = 14 km/s (Geminids is 35 km/s, and Perseids is 59 km/s)” (Table 1).

Activity from 252P/LINEAR

Comet 252P/LINEAR will pass about 0.0357 AU from the Earth on March 21/22[3].

Predictions by Quan-Zhi, Brown & Wiegert

Quan-Zhi’s model for this comet was run from the year 2000 backwards for 100,000 years[5]. The model found a close approach to the Earth (0.10 AU) on 2000 March – which is also when the comet was discovered. Other predicted close approaches, in 1921 and 1847, do not appear in historical records.

The model was also used to compute meteor activity for the 2016 apparition. He writes: “the meteor activity from 252P/LINEAR (Mar 27/28, the observation circumstance again favors observers at South Africa if I remember it right) ... is a bit more promising to see something, though probably no more than a few meteors per hour.” Depending on model parameters (“ejection = low or normal”, which meteoroid trail is considered), the predictions for maximum range from March 27 at 17:09 to March 28 at 02:36. The radiant lies in Lepus (Table 1).

“Cometary and meteor observations during the comet’s unprecedented close approach to the Earth around 2016 Mar. 21 would be useful for the understanding of the surface and evolutionary properties of this unique comet,” Quan-Zhi writes.

They write: “A standard model was developed by integrating the orbit of the comet back to A.D. 1850 and ejecting dust at each perihelion passage since that time. After forward-integrating these particles to Mar. 2016, it was found that at no time this year are the densest dust trail sections in the earth’s path. Instead, a diffuse cloud of perturbed meteoroids ejected during 1894-1926 is calculated to be in the earth’s path during solar longitude 7.5-8.9 degrees (peak at 8.15 deg; equinox J2000.0), between Mar. 28.0 and 29.417 UT. Dust ejected in 1921 is predicted to peak around solar lon[g]itudes 8.01 and 8.47 deg (Mar. 28.5 and 28.958 UT, respectively), while dust from 1915 would peak at 8.27 deg (Mar. 28.75). Slow meteors will radiate from a geocentric radiant at R.A. = 77.0 deg, Decl. = –16.3 deg, with velocity V_g = 11.1 km/s. Rates will be low.”

Summary of predictions

Table 1 lists, chronologically, the date/time of maximum meteor activity predicted for the two comets. The second-last column lists vg (geocentric velocity of the meteors) in km/s; the last distinguishes different assumptions of ejection rates. The first six entries are by Quan-Zhi’s group; the last row is from Jenniskens & Vaubaillon.

How to observe the potential meteors

Tim Cooper, Meteor Specialist of the ASSA Shallow-Sky Section, warns: “There is no guarantee anything will be seen ... but its worth having a look just in case something unexpected happens. ... Given the low geocentric velocity, [the] shower members will be easily recognised.”[7]

In addition to the standard procedures for meteor counting, all meteors noted during these watches need to be plotted on special charts. Tim writes: “The given radiant is tentative, and its actual position, if any shower materialises, needs to be determined accurately. Don’t raise any expectations so that your observers aren’t disappointed. Plan on seeing nothing, or at best a couple of very slow moving meteors. Even seeing no activity is a positive result.”

The graphs in Figure 1 show the altitude of the radiants (as seen from Cape Town) on the date of maximum.

From these diagrams it is clear that conditions are not optimal. P/2016 BA 14 has the best prospects, with the radiant being at an altitude of about 40° at the time of maximum predicted activity. Observe on March 20 from sunset to about 02:00.

252P/LINEAR has very poor prospects. The first predicted maximum occurs during daylight (17:09 SAST). Only the second maximum, at 22:47, can be observed, and then at an altitude of about 20°. The later maxima occur after their radiants have set. On March 27 and March 28, observe from sunset to about midnight.

Bear in mind that the predictions listed in Table 1 have considerable uncertainty, and only observations can help resolve the situation. Tim stresses “the importance of starting observations as soon as possible after dark and observe until the radiant becomes too low.”[8]